Progress dissecting the oral microbiome in caries and health

Abstract

Recent rapid advances in "-omics" technologies have yielded new insights into the interaction of the oral microbiome with its host. Associations of species that are usually considered to be acid-tolerant with caries have been confirmed, while some recognized as health-associated are often present in greater proportions in the absence of caries. In addition, some newly identified bacteria have been suggested as potential contributors to the caries process. In spite of this progress, two major challenges remain. The first is that there is a great deal of heterogeneity in the phenotypic capabilities of individual species of oral bacteria. The second is that the most abundant taxa in oral biofilms display remarkable phenotypic plasticity, i.e., the bacteria associated most strongly with health or with caries can morph rapidly in response to alterations in environmental pH, carbohydrate availability and source, and oxygen tension and redox environment. However, new technologic advances coupled with "old-fashioned microbiology" are starting to erode the barriers to a more complete understanding of oral biofilm physiology and ecology, and in doing so are beginning to provide insights for the creation of novel cost-effective caries control therapies.

Figure.

The principle sources of, and pathways for, pH homeostasis in oral biofilms. Urea is present in all salivary gland secretions and in gingival crevicular fluid at concentrations approximating those in serum (1-10 mM). Arginine is present in low µM quantities in salivary secretions, but is abundant in peptide form. Urease enzymes are encoded by a relatively small number of oral bacteria, mainly Streptococcus salivarius, some Actinomyces spp., and oral Haemophilus spp., and cleave urea into 2 molecules of ammonia and 1 of CO₂. The arginine deiminase pathway (ADS) is a 3-enzyme system that cleaves arginine to ornithine, 2 molecules of ammonia, and 1 of CO₂. The most abundant ADS-positive bacteria are oral Streptococcus spp. and some lactobacilli, including Lactobacillus fermentum. The oral streptococci tend to express the ADS at much higher levels than other oral bacteria.